The Seebeck effect is the conversion of heat directly into
electricity at the junction of different types of wire.

So how does this explain the fact that a thermoelectric current is generated using the same material at a junction.?

The only explanation I can think of is that the Seebeck coefficient (which is a function of the temperature) is the reason for electron flow - but this is just a guess/hunch.

The other thing which left me (as well as the video creator) baffled, is that this particular configuration (same material at junction), generates far more voltage than the "more normal" pairing of say Copper and some other metal (Iron, Aluminium etc).